1. Technical Field of the Invention
The present invention relates to a method and circuit for controlling channels in random accesses by using preamble signals under CDMA in order to allocate efficiently access channels particularly for mobile communication systems.
2. Description of the Prior Art
A control channel is independent upon a communication channel in the conventional mobile communication system. A base station controls mobile stations through the control channels, when a call occurs. Here, a plurality of base stations is provided in a wide service area. Further, a common frequency is used for the communication channels for zones which are distant enough to neglect interference noises. Thus, frequency resource is used efficiently.
The frequency may also be common for the control channel in the zones which are distant enough to neglect interference noises.
In JP 3-6932 A (1991), the mobile station transmits a control signal of which header designates an object base station. On the other hand, the base station transmits a free channel signal for allowing the mobile station to transmit the control signal and an identifer for the base station. When the identifier is identical with that transmitted by the mobile station, the base station stops transmitting the free channel signal, thereby knowing immediately whether or not the mobile station stays in its zone. Thus, the channel is efficiently controlled.
Further, in JP 8-167885, a pseudo carrier pulse is transmitted into a radio channel network, by determining whether any carrier is transmitted or not. Further, a channel number for a data packet is confirmed, and channel allocation is controlled, when a plurality of communication terminals uses a channel competitively. Further, data items from the communication terminals are determined and two-stepped priorities are set up on the basis of the determination result.
Further, in JP 10-178386 A (1998), transmission management unit 44 as shown in FIG. 6 modulates packets with a preamble and data generated by down stream signal generation unit 45. The modulated packets are transmitted through duplexer 42 by antenna 41, in a random access CDMA system. A down stream standard signal which is as long as the preamble may also be transmitted.
Communication terminal 30 receives the down stream standard signal through antenna 31 and duplexer 32 by using receiving unit 33. The downstream standard signal is detected by transmission timing pickup unit 35, thereby executing the random access by using transmission unit 34 on the basis of detected transmission timing signal.
Delay profile measurement unit 47 in base station 40 calculates a correlation between the preamble and a pseudo-noise code in order to obtain a delay profile in the basis of cyclic addition for reducing noises. The timing signal outputted from delay profile measurement unit 47 is inputted into despreading management units 481˜483 which demodulates the received signal by despreading it by using the pseudo-noise code synchronized with the timing signal.
Thus, RAKE reception can be executed by the outputs from despreading management units 481˜483, when the packet with preamble and data is modulated by a short period pseudo-noise. On the other hand, when the packet is modulated by a long period pseudo-noise in the slotted ALOHA system, the delay time can be estimated, the synchronous timing can be acquired, thereby reducing transmission power, separating long delay profiles, and executing the random access.
Further, in JP 2000-59850 A, an additional bandwidth is allocated fairly and efficiently in a cellular communication system for transmitting and receiving data burst between mobile stations.
Furthermore, the conventional CDMA mobile communication system includes a random access system which uses upward random access link channel, or RACH (random access channel).
RACH as shown in FIG. 5 comprises preamble signal 28 and message part 30. Message part 30 is used for transmitting actual information, while preamble signal 28 is used for a request signal for obtaining the message part from a mobile station. In other words, preamble signal is used by mobile stations for requesting a right of using the message part which are commonly used by all the mobile stations.
Therefore, the base station has to allocate the finite message part resource, when a plurality of mobile stations request simultaneously the right of using the message part. In this case, one or more mobile stations are rejected inevitably in the random access system. Therefore, it is desirable to give a priority to the mobile station which was once rejected, in order to avoid a long time wait of that mobile station which was once rejected.
Accordingly, the base station may store the identifiers of the once rejected mobile stations, compare the identifiers with the calling mobile stations, and give a priority to the mobile station of which identifier is stored in the base station. The simplest identifier may be the preamble signal, because its length is constant. However, the preamble can not always be used in every CDMA mobile communication system. For example, the preambles of RACH in W-CDMA expectedly introduced in the year 2001 are only ten or more. The mobile station selects randomly one preamble among the ten or more preambles, when the mobile station transmits the preamble. Therefore, the base station can not identify the mobile station by such preambles. Accordingly, another kind of identifier is required.
The propagation delay time is employed in the present invention, because the propagation delay time depending upon a position and environment of the mobile station is measured correctly in the CDMA system and therefore useful to identify the mobile station.